The present invention relates generally to image enhancement techniques and more particularly to image enhancement techniques which control the dynamic range of signals applied to a display device.
Present day imaging systems, such as thermal imaging systems and the like, generally employ a detector array which provides output signals in a video format. The output signals are processed and applied to a video display for viewing by an operator. As is typically the case, the detector array has a greater dynamic range than the video display. For example, a typical detector might provide output signals which are converted into 256 digitized output levels (shades of gray), while the video display can provide ten output levels (shades of gray). The selection of which output levels from the detector are applied to the video display has a great effect on the quality and clarity of the reproduced image.
Prior art image enhancement techniques have included scaling and histogram equalization. The scaling technique involves spreading of a portion of a scene's amplitude domain linearly over the video display's dynamic range, while allowing the remainder of amplitude domain to saturate. Histogram equalization involves the partitioning of the scene's amplitude domain into amplitude bands which are forced to have the same population distribution. Typical of histogram equalization techniques are those disclosed in U.S. Pat. No. 3,983,320, entitled "Raster Display Histogram Equalization", by Ketcham et al, and U.S. Pat. No. 3,979,555, entitled "Histogram Equalization System for Display Improvement", by Opittek et al.
The Ketcham et al patent discloses a device which performs a sliding window histogram equalization of a television image in real time. This invention computes selected local areas and the histogram is utilized to modify the center or near center elements of the area from which the histogram was computed. Thus, for example, the intensity of a particular point in the image is adjusted according to the histogram of the area contained within a window immediately surrounding the point to be equalized.
The Opittek et al patent discloses a histogram equalization system which utilizes a digital video integrator to address a random access memory. The memory accumulates statistics of the image scene during the addressing cycle. The memory is then read out with the word value accumulated for a portion of the memory equal to the total number of picture elements in the portion of the scene utilized to form the histogram, divided by the number of values which are to be truncated.
The word values determine truncation points which are stored in a latch circuit. The integrator is again read out into truncation logic circuitry which compares each video signal with the truncation point stored in the latching circuitry. Thus, truncated video with a reduced number of video bits is provided as an output. The redistributed intensity levels of the truncate video output have an equal number of picture elements at all intensity levels, resulting in an increase in detail in the region of high occurrence of picture elements.
However, none of the prior art systems is designed in a manner which preserves the gross statistics of the original image scene. The prior art systems generally equalize or manipulate the statistics as opposed to maintaning them.
Accordingly, it would be an improvement in the image processing art to have an image enhancement system which preserves the gross statistics of the image scene while providing an enhanced image.